The environmental authorities are placing ever more stringent demands on the pulp industry to decrease the use of chemicals which can be damaging to the environment, such as, for example, chlorine. Thus, permitted discharges of organic chlorine compounds in the waste water from bleaching plants, following on from the cooking process, have been decreased progressively and are now at such a low level that pulp factories have in many cases stopped using organic chlorine compounds as bleaching agents. In addition, market forces are tending progressively to increase the demand for paper products which are not bleached with chlorine.
The pulp industry is therefore searching for methods which allow bleaching of pulp without using these chemicals. The lignox method (see SE-A 8902058), in which, inter alia, bleaching is carried out with hydrogen peroxide, may be mentioned as an example of such a method. Ozone is another interesting bleaching chemical which is also gaining increased application. It is thus possible, using bleaching chemicals of this nature, to achieve those brightnesses which are required for marketable pulp, i.e. 89 ISO and greater, without using chlorine-containing bleaching agents.
There is, however, a problem in using presently-known bleaching procedures with these bleaching chemicals which do not contain chlorine, namely that they have a significant effect in diminishing the quality of the pulp fibres.
By means of experiments which have been conducted under the auspices of Kamyr AB, it has been found, surprisingly, that extremely good results, with regard to delignification and strength properties, can be obtained if the pulp is cooked at the same temperature level in principally the whole of the digester, i.e. if essentially the same temperature is maintained in all cooking zones, and if a certain quantity of alkali is also supplied to the lowest zone in the digester, which zone is normally used for counter-current washing. Owing to the fact that essentially the same temperature level is maintained in virtually the whole of the digester, very extensive delignification can be achieved at a relatively low temperature. Besides this, it has been found that the strength properties are affected in a particularly favorable manner, that a higher yield of the crude fibre product is obtained and that the quantity of reject material decreases. These advantages are most clearly apparent from the diagrams shown in the FIGS. 1 and 2, which show comparative values between pulp (softwood) which has been cooked using a conventional, modified cooking technique and pulp which has been cooked using the process according to the invention, (in a similar digester, i.e. with a concurrent upper cooking zone, a central counter-current cooking zone and a bottom counter-current washing zone) in which a constant temperature level of about +155 .degree. C. has been maintained in the whole digester.
The invention especially relates to (but not exclusively) an advantageous arrangement of a set of apparatus for achieving a cooking according to the new process such as is disclosed in co-pending U.S. application Ser. No. 08/051,396, filed Apr. 23, 1993, the disclosure of which is incorporated herein by reference. In particular with regard to digesters built according to an older principle, the present invention is also applicable where the process consists of an upper concurrent cooking zone and a lower counter-current washing zone. Such an arrangement is necessary since certain practical problems arise as a consequence of an isothermal cooking process. The first such problem is the difficulty of efficiently reaching and maintaining the temperature in the lower part of the digester, i.e. that part which is normally employed for washing.
The main object is to create a more efficient screening means in order to improve the circulation and as a consequence also the temperature distribution in the digester. In this context it has been found to be advantageous to use digester screening arrangements including circular screens, especially in connection with converting existing digesters, both of the modified type and the older type, for operation according to the new process, but also in connection with building of new digesters.